Heterotrophy-coordinated diazotrophy is associated with significant changes of rare taxa in soil microbiome

Abstract

Soil heterotrophic respiration during decomposition of carbon (C)-rich organic matter plays a vital role in sustaining soil fertility. However, it remains poorly understood whether dinitrogen (N2) fixation occurs in support of soil heterotrophic respiration. In this study, 15N2-tracing indicated that strong N2 fixation occurred during heterotrophic respiration of carbon-rich glucose. Soil organic 15N increased from 0.37 atom% to 2.50 atom% under aerobic conditions and to 4.23 atom% under anaerobic conditions, while the concomitant CO2 flux increased by 12.0-fold under aerobic conditions and 5.18-fold under anaerobic conditions. Soil N2 fixation was completely absent in soils replete with inorganic N, although soil N bioavailability did not alter soil respiration. High-throughput sequencing of the 16S rRNA gene further indicated that: i) under aerobic conditions, only 15.2% of soil microbiome responded positively to glucose addition, and these responses were significantly associated with soil respiration and N2 fixation and ii) under anaerobic conditions, the percentage of responses was even lower at 5.70%. Intriguingly, more than 95% of these responses were originally rare with < 0.5% relative abundance in background soils, including typical N2-fixing heterotrophs such as Azotobacter and Clostridium and well-recognized non-N2-fixing heterotrophs such as Sporosarcina, Agromyces, and Sedimentibacter. These results suggest that only a small portion of the soil microbiome could respond quickly to the amendment of readily accessible organic C in a fluvo-aquic soil and highlighted that rare phylotypes might have played more important roles than previously appreciated in catalyzing soil C and nitrogen turnovers. Our study indicates that N2 fixation could be closely associated with microbial turnover of soil organic C when available in excess.